Mineral coal, frequently referred to also as hard coal, pit coal or Steinkohle, is extracted from the mining shafts and tunnels generally accomplished by noncombustible or combustion-impeding impurities including pyrites and other mineral matter referred to hereinafter as mining detritus.
Various method have been proposed to separate these impurities from the coal, especially since pyrites contribute a large portion of the sulfur found in the mined product.
The pyrites are generally in the form of iron disulfides (iron pyrite) and contain up to 50% and more by weight of sulfur. In such coal mined in the Ruhr valley, for example, some 40 to 60% of the sulfur content is in the form of pyrites .
The pyrite crystals, generally in pure form, are locked into the coal structure together with other mineral matter or, conversely, the coal may be interlaced in the structure of other mineral matter containing such pyrites.
Removal of the pyrites from such mineral coal is important prior to combustion to reduce the sulfur content of the combustion gases and hence the environmental contamination thereby.
In the methods proposed heretofore for the removal of pyrites from coal, use has been proposed of the magnetic characteristics of the pyrite. The mined product is subjected to magnetic fields which separate the pyrite from the coal. Such systems require expensive apparatus and are energy intensive. In fact, these methods are still under investigation and have not been applied on a large scale in a continuous manner successfully hitherto.
Another disadvantage of the magnetic separation approach is that it does not remove mineral detritus other than pyrites from the coal.
The methods which have been successfully applied heretofore on a large scale are wet methods utilizing hydrocylones, flotation processes and the like which have a further advantage in that they allow classification of the coal for its various marketing possibilities.
While these methods have been improved upon in recent years and have played a significant role in the reduction of the sulfur content in the combustion of coal, they are not universally applicable and involve certain disadvantages including that of recovering the coal in a wet state and difficulties in handling the wet product.